The present invention relates to photometric devices and, more particularly, to photometric devices for determining the optimum film exposure interval for a camera in which the exposure interval is corrected for differences in illuminance between the subject of principal interest in the central portion of the scene to be photographed and the subjects of secondary interest in the foreground and background portions of the scene.
Various types of single-cell and multi-cell photometers have been developed to measure the light reflected from a scene to be photographed to compute or otherwise determine the film exposure interval for the camera.
The simplest photometers generally include a single photo-responsive cell that has a field of view approximately the same as that of the camera. The cell provides an electrical output responsive to the average light reflected from the entire scene to be photographed. While this type of photocell arrangement generally provides accurately determined exposure intervals for a narrow range of illumination, the accuracy of the exposure determination decreases as the range of brightness or illumination over the scene increases. For example, the exposure determination derived from a single-cell averaging photometer can be adversely affected by a bright source of light occupying a relatively small area of the scene such as a small, but bright lamp in the foreground portion of an interior scene. In order to overcome this type of disadvantage, single-cell spot meters, which have a field of view covering a fraction of the total scene area, have been developed to permit the exposure determination to be made for the subject of primary interest while generally ignoring the foreground and background portions of the scene. The use of a spot meter, however, requires that the photographer be sufficiently skilled to accurately aim the spot meter at the subject of principal interest, and, when photographing a scene without a subject of primary interest, e.g., a long distance landscape scene, to manually scan the scene to ascertain the maximum and minimum brightnesses to estimate an acceptable average film exposure interval.
Various types of multi-cell photometers have been developed in an effort to increase the accuracy of the film exposure interval determination over a wide range of scene illumination. In some of these multi-cell photometers, the individual cells are arranged in an ordered array of rows and columns occupying a field of view equivalent to that of the camera with the individual outputs of the cells electronically evaluated to determine the maximum and minimum brightness and to provide an exposure determination based on these outputs. In yet more sophisticated photometers, the cells are arranged having a central portion that coincides with the probable subject of principal interest in the central portion of the scene and a peripheral portion that surrounds the central portion, for example, as disclosed in U.S. Pat. Nos. 4,005,444; 3,884,584; and 3,836,920.
As photometric devices have become more sophisticated, the amount of electronic circuitry necessary to determine the exposure interval has increased dramatically, and, as a practical result, sophisticated photometric devices for use with contemporary cameras are relatively complex electronic devices requiring extensive electronic circuitry. The cost of these sophisticated photometers is such that their use is generally restricted to more expensive cameras rather than inexpensive cameras intended for the mass consumer market.